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Date: Thu, 19 May 2011 21:55:53 +0800
From: Hillf Danton <dhillf@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
Peter Zijlstra <peterz@...radead.org>,
Yong Zhang <yong.zhang0@...il.com>
Subject: Re: [PATCH] sched: change pull_rt_task() to decrease time waiting on runqueue
On Wed, May 18, 2011 at 11:20 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed, 2011-05-18 at 22:54 +0800, Hillf Danton wrote:
>
>> In short, if there are pushable tasks and if there are RQs,
>> NOT LIMITED TO our RQ, in lower priority, tasks should be pushed to
>> RQs as many as we could.
>
> I understand what you are trying to do. But this change modifies a lot
> of assumptions. Please supply test cases that shows how this helps.
>
> Have a look at:
>
> http://lwn.net/Articles/425583/
>
> Where I did a bit of work just to make sure my change to sched_rt.c was
> appropriate. Just coming up with scenarios may not be good enough.
> Seeing it in practice is worth much more.
>
> For example, you may be making the fast path slower. This may do what
> you expect, with a hit in performance. I'm not sure I like that idea.
>
Hi Steve
The patch is updated with a few changes.
A clone of find_lowest_rq(), find_lowest_rq_for_pushing() is added to select
the best RQ for a given pushable tasks, in a manner that the selected RQ
has to be different from the task's RQ.
It is not the final version, since no test cases check it, but a
target for comments.
Thank you very much for sharing your work.
Hillf
---
--- a/kernel/sched_rt.c 2011-04-27 11:48:50.000000000 +0800
+++ b/kernel/sched_rt.c 2011-05-19 21:26:42.000000000 +0800
@@ -1260,6 +1260,28 @@ static int find_lowest_rq(struct task_st
return -1;
}
+static int find_lowest_rq_for_pushing(struct task_struct *task)
+{
+ struct sched_domain *sd;
+ struct cpumask *lowest_mask = __get_cpu_var(local_cpu_mask);
+ int cpu = task_cpu(task);
+
+ if (task->rt.nr_cpus_allowed == 1 ||
+ !cpupri_find(&task_rq(task)->rd->cpupri, task, lowest_mask))
+ return -1;
+
+ for_each_domain(cpu, sd) {
+ int best_cpu;
+ best_cpu = cpumask_first_and(lowest_mask,
+ sched_domain_span(sd));
+
+ if (best_cpu < nr_cpu_ids && best_cpu != cpu)
+ return best_cpu;
+ }
+
+ return -1;
+}
+
/* Will lock the rq it finds */
static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
{
@@ -1268,7 +1290,7 @@ static struct rq *find_lock_lowest_rq(st
int cpu;
for (tries = 0; tries < RT_MAX_TRIES; tries++) {
- cpu = find_lowest_rq(task);
+ cpu = find_lowest_rq_for_pushing(task);
if ((cpu == -1) || (cpu == rq->cpu))
break;
@@ -1336,6 +1358,7 @@ static int push_rt_task(struct rq *rq)
{
struct task_struct *next_task;
struct rq *lowest_rq;
+ int ret = 0;
if (!rq->rt.overloaded)
return 0;
@@ -1383,7 +1406,6 @@ retry:
* since the other cpus will pull from us when they
* are ready.
*/
- dequeue_pushable_task(rq, next_task);
goto out;
}
@@ -1402,7 +1424,7 @@ retry:
deactivate_task(rq, next_task, 0);
set_task_cpu(next_task, lowest_rq->cpu);
activate_task(lowest_rq, next_task, 0);
-
+ ret = 1;
resched_task(lowest_rq->curr);
double_unlock_balance(rq, lowest_rq);
@@ -1410,7 +1432,7 @@ retry:
out:
put_task_struct(next_task);
- return 1;
+ return ret;
}
static void push_rt_tasks(struct rq *rq)
@@ -1423,77 +1445,20 @@ static void push_rt_tasks(struct rq *rq)
static int pull_rt_task(struct rq *this_rq)
{
int this_cpu = this_rq->cpu, ret = 0, cpu;
- struct task_struct *p;
- struct rq *src_rq;
+ int count = 0;
if (likely(!rt_overloaded(this_rq)))
return 0;
+again:
for_each_cpu(cpu, this_rq->rd->rto_mask) {
- if (this_cpu == cpu)
- continue;
-
- src_rq = cpu_rq(cpu);
-
- /*
- * Don't bother taking the src_rq->lock if the next highest
- * task is known to be lower-priority than our current task.
- * This may look racy, but if this value is about to go
- * logically higher, the src_rq will push this task away.
- * And if its going logically lower, we do not care
- */
- if (src_rq->rt.highest_prio.next >=
- this_rq->rt.highest_prio.curr)
- continue;
-
- /*
- * We can potentially drop this_rq's lock in
- * double_lock_balance, and another CPU could
- * alter this_rq
- */
- double_lock_balance(this_rq, src_rq);
-
- /*
- * Are there still pullable RT tasks?
- */
- if (src_rq->rt.rt_nr_running <= 1)
- goto skip;
-
- p = pick_next_highest_task_rt(src_rq, this_cpu);
-
- /*
- * Do we have an RT task that preempts
- * the to-be-scheduled task?
- */
- if (p && (p->prio < this_rq->rt.highest_prio.curr)) {
- WARN_ON(p == src_rq->curr);
- WARN_ON(!p->se.on_rq);
-
- /*
- * There's a chance that p is higher in priority
- * than what's currently running on its cpu.
- * This is just that p is wakeing up and hasn't
- * had a chance to schedule. We only pull
- * p if it is lower in priority than the
- * current task on the run queue
- */
- if (p->prio < src_rq->curr->prio)
- goto skip;
-
- ret = 1;
+ if (cpu != this_cpu)
+ count += push_rt_task(cpu_rq(cpu));
+ }
- deactivate_task(src_rq, p, 0);
- set_task_cpu(p, this_cpu);
- activate_task(this_rq, p, 0);
- /*
- * We continue with the search, just in
- * case there's an even higher prio task
- * in another runqueue. (low likelihood
- * but possible)
- */
- }
-skip:
- double_unlock_balance(this_rq, src_rq);
+ if (ret != count) {
+ ret = count;
+ goto again;
}
return ret;
--
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